Question 864390
x for length
y for width

x=2y, and 2(x+y)=84.
Substitution gives {{{2(2y+y)=84}}}
{{{2*3y=84}}}
{{{6y=84}}}
{{{y=84/6}}}
{{{highlight(y=14)}}}
-
{{{highlight(x=28)}}}